Cyclic phosphodiesterases are intracellular enzymes which, through the hydrolysis of cyclic nucleotides cAMP and cGMP, regulate the levels of these mono phosphate nucleotides which serve as second messengers in the signaling cascade of G-protein coupled receptors. In neurons, PDEs also play a role in the regulation of downstream cGMP and cAMP dependent kinases which phosphorylate proteins involved in the regulation of synaptic transmission and homeostasis. To date, eleven different PDE families have been identified which are encoded by 21 genes. The PDEs contain a variable N-terminal regulatory domain and a highly conserved C-terminal catalytic domain and differ in their substrate specificity, expression and localization in cellular and tissue compartments, including the CNS.
The discovery of a new PDE family, PDE10, was reported simultaneously by three groups in 1999 (Soderling et al. “Isolation and characterization of a dual-substrate phosphodiesterase gene family: PDE10A” Proc. Natl. Sci. 1999, 96, 7071-7076; Loughney et al. “Isolation and characterization of PDE10A, a novel human 3′,5′-cyclic nucleotide phosphodiesterase” Gene 1999, 234, 109-117; Fujishige et al. “Cloning and characterization of a novel human phosphodiesterase that hydrolyzes both cAMP and cGMP (PDE10A)” J. Biol. Chem. 1999, 274, 18438-18445). The human PDE10 sequence is highly homologous to both the rat and mouse variants with 95% amino acid identity overall, and 98% identity conserved in the catalytic region.
PDE10 is primarily expressed in the brain (caudate nucleus and putamen) and is highly localized in the medium spiny neurons of the striatum, which is one of the principal inputs to the basal ganglia. This localization of PDE10 has led to speculation that it may influence the dopaminergic and glutamatergic pathways both which play roles in the pathology of various psychotic and neurodegenerative disorders.
PDE10 hydrolyzes both cAMP (Km=0.05 uM) and cGMP (Km=3 uM) (Soderling et al. “Isolation and Characterization of a dual-substrate phosphodiesterase gene family: PDE10.” Proc. Natl. Sci. USA 1999, 96(12), 7071-7076). In addition, PDE10 has a five-fold greater Vmax for cGMP than for cAMP and these in vitro kinetic data have lead to the speculation that PDE10 may act as a cAMP-inhibited cGMP phosphodiesterase in vivo (Soderling and Beavo “Regulation of cAMP and cGMP signaling: New phosphodiesterases and new functions,” Curr. Opin. Cell Biol., 2000, 12, 174-179).
PDE10 is also one of five phosphodiesterase members to contain a tandem GAF domain at their N-terminus. It is differentiated by the fact that the other GAF containing PDEs (PDE2, 5, 6, and 11) bind cGMP while recent data points to the tight binding of cAMP to the GAF domain of PDE10 (Handa et al. “Crystal structure of the GAF-B domain from human phosphodiesterase 10A complexed with its ligand, cAMP” J. Biol. Chem. 2008, May 13th, ePub).
PDE10 inhibitors have been disclosed for the treatment of a variety of neurological and psychiatric disorders including Parkinson's disease, schizophrenia, Huntington's disease, delusional disorders, drug-induced psychoses, obsessive compulsive and panic disorders (US Patent Application 2003/0032579). Studies in rats (Kostowski et. al “Papaverine drug induced stereotypy and catalepsy and biogenic amines in the brain of the rat” Pharmacol. Biochem. Behav. 1976, 5, 15-17) have showed that papaverine, a selective PDE10 inhibitor, reduces apomorphine induced stereotypies and rat brain dopamine levels and increases haloperidol induced catalepsy. This experiment lends support to the use of a PDE10 inhibitor as an antipsychotic since similar trends are seen with known, marketed antipsychotics.
Antipsychotic medications are the mainstay of current treatment for schizophrenia. Conventional or classic antipsychotics, typified by haloperidol, were introduced in the mid-1950s and have a proven track record over the last half century in the treatment of schizophrenia. While these drugs are effective against the positive, psychotic symptoms of schizophrenia, they show little benefit in alleviating negative symptoms or the cognitive impairment associated with the disease. In addition, drugs such as haloperidol have extreme side effects such as extrapyramidal symptoms (EPS) due to their specific dopamine D2 receptor interaction. An even more severe condition characterized by significant, prolonged, abnormal motor movements known as tardive dyskinesia also may emerge with prolonged classic antipsychotic treatment.
The 1990s saw the development of several new drugs for schizophrenia, referred to as atypical antipsychotics, typified by risperidone and olanzapine and most effectively, clozapine. These atypical antipsychotics are generally characterized by effectiveness against both the positive and negative symptoms associated with schizophrenia, but have little effectiveness against cognitive deficiencies and persisting cognitive impairment remain a serious public health concern (Davis, J. M et al. “Dose response and dose equivalence of antipsychotics.” Journal of Clinical Psychopharmacology, 2004, 24 (2), 192-208; Friedman, J. H. et al “Treatment of psychosis in Parkinson's disease: Safety considerations.” Drug Safety, 2003, 26 (9), 643-659). In addition, the atypical antipsychotic agents, while effective in treating the positive and, to some degree, negative symptoms of schizophrenia, have significant side effects. For example, clozapine which is one of the most clinically effective antipsychotic drugs shows agranulocytosis in approximately 1.5% of patients with fatalities due to this side effect being observed. Other atypical antipsychotic drugs have significant side effects including metabolic side effects (type 2 diabetes, significant weight gain, and dyslipidemia), sexual dysfunction, sedation, and potential cardiovascular side effects that compromise their clinically effectiveness. In the large, recently published NIH sponsored CATIE study, (Lieberman et al “The Clinical Antipsychotic Trials Of Intervention Effectiveness (CATIE) Schizophrenia Trial: clinical comparison of subgroups with and without the metabolic syndrome.” Schizophrenia Research, 2005, 80 (1), 9-43) 74% of patients discontinued use of their antipsychotic medication within 18 months due to a number of factors including poor tolerability or incomplete efficacy. Therefore, a substantial clinical need still exists for more effective and better tolerated antipsychotic mediations possibly through the use of PDE10 inhibitors.